The Arun is a river in the English county of West Sussex. Its source is a series of small streams (known locally as gills) in the St Leonard's Forest area located to the east of Horsham.

It flows through Arundel and past the castle, and meets the sea at Littlehampton. Its main tributary is the western River Rother.

The Arun local government district in West Sussex is named for it.

The river runs for approximately 18.75 miles from source to sea, and is one of the faster flowing in England.

Historically, the Arun was known as the Tarrant, but was renamed after Arundel by antiquarians in a back-formation.

The mouth of the river has not always been at Littlehampton. Until the later fifteenth centuary it joined the River Adur at Lancing some ten miles to the east before entering the sea. This estuary became blocked with shingle by the eastward drift of the tides, pushing the Adur towards Shoreham, while the Arun broke out at Worthing, Goring and Ferring at various times, until it formed it's present estuary at Littlehampton between 1500 and 1530.

The Arun was linked to the canal network by the Wey & Arun Canal, opened in 1816 and closed in 1871, which is currently being renovated.

The river had been made navigable to New Bridge on the A272 between Wisborough Green and Billingshurst by 1787, by the Arun Navigation Company.

The Rother Navigation joined the Arun near Stopham Bridge, a fine medieval stone bridge built in AD 1423, which carried the heavy traffic on the A283 until a new bridge was built beside it in the 1980s. The central arch was raised in 1822 to allow masted barges to pass.